1. Field of the Invention
This invention relates to the forming and marking of sheets of plastic material, and more particularly to an improved method of and apparatus for marking a continuous moving sheet of substantially rigid thermoplastic synthetic resin material at spaced intervals along its length whereby individual sheets of material cut from the continuous length are each marked at a predetermined position.
2. Description of the Prior Art
It is conventional practice to form thermoplastic synthetic resin materials such as, for example, polystyrene, acrylic, polycarbonate, and ABS, hereinafter generally referred to as plastic, into sheets of predetermined size by initially extruding, casting, or otherwise forming a continuous sheet of the material into the desired thickness and width. The term "sheets" as used herein generally refers to substantially rigid or self-sustaining sheets and may have a thickness of from about 1/32 inch to 1/4 inch or greater. As the continuous sheet is conveyed from the forming apparatus and the plastic material hardens sufficiently to be substantially self-sustaining, it is cut into individual sheets of the desired size. The continuous sheet can have a width which is substantially greater than the width of the individual sheets cut therefrom, with the continuous sheet being side-trimmed and slit into a plurality of separate continuous lengths which move to the cutting apparatus in edge-to-edge, coplanar relation. For simplicity of description, the term "extrusion" will hereinafter sometimes be employed to include various forming methods and apparatus.
A movable hot knife unit is conventionally used on an extrusion line to cut the continuous lengths of plastic sheet into the desired lengths. The hot knife unit reciprocates along the line of movement of the continuous sheet and is coordinated with the sheet movement so that it is moving in the same direction and at the same speed as the sheet during the cutting operation, thereby avoiding sheet deformation during the time that the hot knife is in contact with the sheet.
It is often desired or necessary to permanently mark plastic sheets for identification as to material, thickness, manufacturer, safety specifications and the like. This is particularly true when the plastic sheets are to be used in architectural applications or as safety shields, where such markings may be required on each sheet. In the past, it has been the conventional practice to apply such markings adjacent one edge the individual sheet with a hot stamp die. This marking, or branding, has conventionally been a separate operation performed after the plastic sheets are cut to size. Such a separate marking operation has necessarily increased the overall cost of the sheet, due to the necessity for additional handling, and the extra operation has resulted in an increase in damaged and rejected sheets.